1. Field of the Invention
The invention relates to hydrostatic transmission systems and use of such a hydrostatic transmission system in powering a winch.
2. Background Art
Well logging involves recording data related to one or more characteristics of subterranean formations penetrated by a borehole. Many types of well logs are recorded by appropriate downhole instruments placed in a housing called a sonde. In wireline-conveyed well logging, the sonde is lowered into the borehole by means of an armored electrical cable wound on the drum of a winch. The measurements are made as the sonde passes the various formations, and data signals are transmitted through the cable to an acquisition system at the earth""s surface. Generally, the sonde can be lowered quickly into the borehole and retrieved quickly from the borehole when it is not acquiring data. However, the physics and design of some well logging instruments require that the sonde be moved precisely inside the borehole, usually at a low speed, while acquiring data. A drive system is used to drive the winch drum so that the cable to which the sonde is attached can be payed out or retrieved at desired speeds. Typically, the drive system includes a prime mover, such as an internal combustion engine, and a hydrostatic transmission system for transmitting power from the prime mover to the winch drum.
Conventional hydrostatic transmission systems include a hydraulic pump, usually a variable-displacement hydraulic pump, and a hydraulic motor, usually a variable-displacement hydraulic motor. The hydraulic pump transfers power from the prime mover to the hydraulic motor, and the hydraulic motor in turn applies a torque to the winch drum. In operation, the prime mover is typically set to operate at a predetermined speed via an engine throttle or the like. The speed of the winch drum is then regulated by controlling the displacements of one of the hydraulic elements of the hydrostatic transmission system. Conventional hydrostatic transmission systems, however, tend to become unstable when the flow rate of the hydraulic pump and the rotational speed of the hydraulic motor are reduced to accommodate very low drum speeds. Such instability may affect the quality of the measurements made by the logging instrument as well as degrade the service performance of the hydrostatic transmission system. As a result, logging speeds have typically been limited to the stable range of the hydrostatic transmission system.
Several solutions have been proposed for overcoming instability of the hydrostatic transmission system at low speeds. For example, U.S. Pat. No. 5,355,675 issued to Mayhugh et al. discloses a stable closed hydrostatic rotary power transmission system which can be used to transmit power from an engine to a winch. The hydrostatic rotary power transmission system disclosed in the Mayhugh et al ""675 patent has a wide dynamic speed control range and includes a hydrostatic motor, a variable-displacement hydraulic pump, and a two-position control device. The transmission system may operate in one of two modes: normal mode and fine-speed control mode. In the normal mode, the two-position control device interconnects the pump with the motor through a first circuit, and the operation of the motor speed is a function of the displacement of the pump. In a fine speed control mode, the two-position control device disables the first circuit configuration and interconnects the pump with the motor through a second circuit. In this mode, the motor speed is a function of the flow rate through a proportional-flow valve downstream of the motor.
One aspect of the invention is a hydrostatic transmission system for driving a rotatable member, which includes a differential having an output rotatably coupled with the rotatable member. The hydrostatic transmission system further includes a first hydrostatic drive operatively coupled to a first input of the differential and a second hydrostatic drive operatively coupled to a second input of the differential. The output of the first hydrostatic drive is rotatable in a first selected direction and at a first selected speed, the output of the second hydrostatic drive is rotatable in a second selected direction and at a second selected speed. A rotational speed and direction of the output of the differential is related to the algebraic sum of the rotational speeds and directions of the outputs of the two hydrostatic drives.
Another aspect of the invention is a method for driving a rotatable member which includes controlling a first hydrostatic drive to a first input of a differential at a selected speed in a first selected direction, and controlling a second hydrostatic drive to rotate a second input of a differential at a second selected speed and in a second selected direction. The first and second speeds and directions are selected to provide a selected output speed and rotation direction of the differential, and consequently, the rotatable member, which is rotatably coupled to the output of the differential.
Another aspect of the invention is a winch system which comprises a rotatably supported drum, a differential having an output coupled to the rotatably supported drum, a first hydrostatic drive having an output coupled to a first input of the differential, and a second hydrostatic drive having an output coupled to a second input of the differential. The output of the first hydrostatic drive is rotatable in a first selected direction, the output of the second hydrostatic is rotatable in a second selected direction, and a rotational speed of the output of the differential is related to algebraic sum of the rotational speeds and directions of the output of the hydrostatic drives.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.